Again, sorry you're having problems with the gun. Just so you know there is no cutter, the nail breaks when enough force is applied, this is where the "pop" comes from.
Just for the record, take the tool back to Lowes with a few pop rivets and ask them to show you how it works. You may find out you have a bad gun, or that you just need to exert much more pressure than you were to pop them. Either way you will gain insight/knowledge, you'll also learn about how long the rivet should be for the thickness of the material.
Rivets are used in a multitude of materials. Just look at your belt and you'll see them there even. Granted not pop rivets, but rivets nonetheless. Rivets are very strong when done correctly and can be used on a multitude of materials.
Again sorry you're having problems with them, I've used them with great success so it's very confusing to me why they would not be working.